1. Field of the Invention
The present invention relates to a method for producing a target substance by using a bacterium. More specifically, the present invention relates to a method for improving production of a target substance such as an L-amino acid, nucleic acid, antibiotic, vitamin, growth factor or physiologically active substance by using a bacterium.
2. Description of the Related Art
The production of target substances such as L-amino acids by fermentation using microorganisms includes the following methods: a method of using a wild-type microorganism (wild-type strain), a method of using an auxotrophic strain derived from a wild-type strain, a method of using a metabolically-regulated mutant derived from a wild-type strain as one of various drug resistant mutants, a method of using a strain having characteristics of both of an auxotrophic strain and a metabolically-regulated mutant and so forth.
In recent years, recombinant DNA techniques have been used for the production of target substances by fermentation. For example, the ability of microorganisms to produce L-amino acids can be improved by enhancing a gene encoding an L-amino acid biosynthesis enzyme (U.S. Pat. Nos. 5,168,056 and 5,776,736), or by enhancing inflow of a carbon source into an L-amino acid biosynthesis system (U.S. Pat. No. 5,906,925).
Methods for improving the production of target substances in a microorganism include methods of modifying the uptake or excretion system of a substance. Examples of a method for modifying an uptake system include a method for improving a target substance-producing ability by deleting or degrading a system for uptake of the target substance into a cell. Specifically, a method of deleting the gluABCD operon or a part thereof to delete or degrade an L-glutamic acid uptake system (EP 1 038 970 A1), a method of attenuating uptake of purine nucleosides into a cell to enhance the purine nucleoside-producing ability (EP 1 004 663 A1), and so forth are known.
Methods for modifying an excretion system of a microorganism include a method of enhancing an excretion system for a target substance and a method of deleting or attenuating an excretion system for an intermediate or substrate of a biosynthesis system of a target substance. As the method of enhancing an excretion system of a target substance, for example, a method for producing L-lysine by using a Corynebacterium bacterium strain in which expression of an L-lysine excretion gene (lysE) is enhanced (WO97/23597) has been disclosed. As for the latter method, a method is known for producing L-glutamic acid as a target substance, in which excretion of α-ketoglutaric acid, an intermediate of the target substance, is reduced by mutating or disrupting the α-ketoglutarate permease gene (WO01/005959).
Furthermore, it has been suggested that the gene encoding the ATP binding cassette superfamily (ABC transporter) involved in permeation of substances through a cell membrane is used for the breeding of microorganisms in which amino acid transport through the cell membrane is modified (WO00/37647).
Furthermore, it has been suggested that the gene encoding sucrose PTS enzyme II, a protein involved in the uptake of sucrose into a cell, is used in a coryneform bacterium for breeding of a strain exhibiting improved productivity of an amino acid, nucleic acid etc. (EP 1 197 555 A). Furthermore, a technique for improving L-amino acid productivity of a bacterium belonging to the genus Escherichia using a sucrose PTS gene group or a sucrose non-PTS gene group (U.S. Patent Application No. 2001/0049126) is also known.
A technique for reducing production of a byproduct of a target substance by deleting or attenuating the biosynthesis system of the byproduct (for example, “Amino Acid Fermentation” Gakkai Shuppan Center, p. 4, 1986) is known. However, in this method, when a microorganism is cultured, the aforementioned byproduct needs to be added to a medium in an amount necessary for growth.
Mtr (Heatwole, V. M. et al., J. Bacteriol., American Society for Microbiology, 173, pp. 108-115, January 1991) and TnaB (Sarsero, J. P. et al., J. Bacteriol., American Society for Microbiology, 173(10), pp. 3231-3234, May 1991) are known as L-tryptophan-specific uptake systems, and AroP (Mee-Len, C. et al., J. Bacteriol., American Society for Microbiology, 167(2), pp. 749-753, August 1986) is known as an uptake system common to aromatic amino acids. However, it has not been previously described to improve productivity of a target substance by enhancing an uptake system for a byproduct of the target substance.